


Dedication to Valentine's

by StarryBlanket



Category: Undertale (Video Game)
Genre: Alternate Universe - Swapfell (Undertale), Alternate Universe - Underswap (Undertale), Blueberry (Underswap) - Freeform, Breeding, Fluff, Fontcest, Incest, M/M, Mating Cycles/In Heat, Oviposition, Sanscest - Freeform, Smut, Swapfell Papyrus (Undertale), Swapfell Sans (Undertale), Underswap Papyrus (Undertale), Underswap Sans (Undertale)
Language: English
Status: In-Progress
Published: 2019-02-18
Updated: 2019-02-18
Packaged: 2019-10-31 02:09:10
Rating: Mature
Warnings: Graphic Depictions Of Violence, Rape/Non-Con
Chapters: 1
Words: 2,169
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/17840390
Author URL: https://archiveofourown.org/users/StarryBlanket/pseuds/StarryBlanket
Summary: I'll be making about four stories based off the votes in the polls on my Twitter. They're one-shot stories. If you guys really like it, leave a comment, enjoy ;)





	Dedication to Valentine's

**Author's Note:**

> After a fight led to the brothers being stuck at an odd universe, Slim gets attacked by lamia. Carefully taken home, Black blames himself for everything. The skeleton brothers goes through many changes, one change in particular will change everything.

It’s been a week since they’ve entered that fucked up universe. The Swapfell brother's didn’t mean to but were distracted by their argument and went through the wrong portal. While Black decided to wonder off leaving his mutt alone, he called for Ink. Slim wasn’t able to protect himself alone from a lamia attacking him and injecting him with some weird liquid.

Black saved his brother but couldn’t save from the awful life changing illness that was cursed upon Slim. Slim stood in bed for a whole week going through mental and physical changes. His legs formed into a thick burnt orange ecto tail, his teeth grew sharper and would occasionally drip a dark liquid, and he became more possessive and angry.

Black almost quit his job to take care of Slim. He wasn’t in good shape by the end of the first week. He wasn’t sleeping, eating, and doing well at work. If it wasn’t for Alphys, he would have had his ass beat by Burgerpants and his gang.

* * *

By the second week, Slim began to return to his senses. He would calm down when needed and actually listen to Black. While this was going on, Ink looked for a cure. Slim began the same thing that attacked him, a lamia. This was all because of Black. He blamed himself so harshly. He started the argument, he left Slim, he let him become sick. This was a huge downfall in his life.

Slim was shocked at first. When he came back to his senses, he freaked out. He hated himself. He wanted his legs back! He didn’t want to be that monster. He wanted to walk with his Lord and bow to him on his knees! There was something telling him that this was his brothers fault. But it wasn't! If Black had just stopped fighting and stood by his brother's side This wouldn’t have happened. No. His Lord tried to save him.

He could tell Black wasn’t doing well. Slim knew that he was trying his hardest to not reveal his feelings. Black even started reading his own books to Slim and checking on him every ten minutes; even at night. As much as Slim loved it, he knew it was because of Black's anxiety.

Undyne came over to check on Slim. She was like a frozen fish when she saw her best friend. He was like a whole new monster. The changes were so new to her eyes. She tried thinking of ways to fix what’s been done but couldn’t really figure anything out. She did bring over some medication for Black's sleep and anxiety.

The second week was easier. Black was doing better with the medication and was sleeping well. He was focusing on himself more than before. Slim was glad to know that his brother was doing fine. It was good to see his old Lord back in business. 

* * *

By the third week, Black's medication ran out. He was going off track again. He was worrying about Slim who was finally roaming the house freely with his new tail. He comforted Black every time he saw the self hate surface from his soul. 

Ink told them that there was no fixing this. He couldn’t find anything. The guardian said he will keep looking but to not have their hopes up. It really broke Black. He actually cried in front of Slim and Ink. The two frowned at the sadness coming from the small Sans. Slim hugged him, he hugged him tightly. There was an urge to snap his brothers neck. 

Everyday Black would work. They began to have financial issues without Slim working. Black was working overtime to pay the bill as there was little money for food. The food he would buy was really for Slim, not for himself. Slim was starting to hate the way Black was acting. He was hating this whole damn situation. 

* * *

Week four was rough, really rough. Slim was finally slithering around Snowdin and exploring the fresh land. Black made sure he was okay. Slim was very aggressive towards everyone. His brother had to stop him from starting fights. There wasn’t even a reason to start the fights. Just the glances woukd set the lamia off. 

Slim punched Black for stopping him. This was a different kind of strength than Slim’s original strength. Black gained a crack along his other eye socket and it bled. They both looked at each other in shock before going home quietly. Black held his eye socket the hole time, even as blood and dust dripped through his fingers.

It felt good to have that power. To have the power to stop his brother, to control him. He was the one in control of that situation, not Black. He defied his master's order and his master did nothing. It felt amazing.

* * *

Week five came slowly. There was no news for a cure. Black was rarely home anymore, too busy working. He wouldn’t sleep for a couple days at times. Alphys finally told Black to take a few days off. Slim would usually find that little skeleton passed out in the couch, an alcoholic beverage on the floor. He started drinking and Slim hated it.

His heat had came up. He didn’t know what to do with his heat in such a body. He didn’t want to do anything. The way his little Lord was sitting on the couch tough. He was wearing a baggy t-shirt and shorts. His small brother was so beautiful. 

Going up to him, he stood in the way if the T.V. and Black. Black looked up at Slim, “What are you doing?”

Slim leaned close to his brother and licked his neck, “I’m in heat M’lord.” 

“You know I can’t help you.. Not when you’re like this. We don’t know how your heat cycles work or what you could do.”

“Then let’s find out,” Slim picked up the Sans and went upstairs. He ignore the fussing coming from his little mate. He will teach him some manners one day.

Throwing Black on Slim’s bed, the lamia started taking off the smaller's clothes. 

“Stop That! I didn’t not give you such permission to touch me!”

It was getting really annoying now. He punched Black's arm. He fractured his brother's arm. Black cried out in pain and begged for Slim to let him go. 

No.

He stripped Black of his clothes and viewed his beautiful bones. He ignored the broken one. That was Black's fault.

Slim began licking Blacks neck and rocking his hips against his brother. Black spread his legs opened but refused to summon anything. As much as he didn’t want to do this, he did owe his brother for being in thjs situation. He was being stimulated to much, it felt so different and good.

Black tried pushing him away. He was pissed and about his summon a weapon until Slim summoned his soul and grabbed it. Black screamed as Slim dug his fangs right into the center of Black’s soul. A warm liquid oozed into the soul. It felt so good but he also felt his body go slack.

With all the grinding and humping, Black's pussy was finally summoned. It was already dripping wet with the sweet scent of arousal. Slim pushed his face against his brother's sweetness and licked his folds. Each time the tongue touched the clit, he let out a long moan. It felt so good.

The tongue soon went right in him and right out. It was so long and thick. It was beautiful. The cries of pleasure and the wet noises. It was like music to Slim’s ears.

Slim didn’t let his brother have his orgasm. Black cried from the loss of contact. Two ecto dicks came out of a hidden part of Slim’s magic. Black gasped and stared at both of them. They were **huge**. He could rarely handle one of his brother's dick; now there was two.

“You look so delicious~ So interested~” Slim hissed and grinded his twin dicks against Black's small pussy.

“P-papyrus.. You have to stop. You’re not going to fit. You’ll tear me apart.” 

“Maybe that’s what I plan to do, M'lord.” He smirked as he saw Black's eyes widen. Slim brushed both of his tips at Black's entrance and began to push in. The Sans wanted to move, to fight, to do anything to get away.

Slim stopped and laughed. He pulled out the tips and laughed at Black whose face was turning a dark shade of red from humiliation. “I won’t break you, not yet.” He saw how pitiful his brother looked and loved it. He was in charge now.

Slim decided to use the dick that felt right to use. Only giving Black a little mercy, he thrusts right into his pussy. The little skeleton yelped out from the intrusion. He began thrusting at a fast pace, his other dick grinding against Black's ass.

There was pleasure, so much pleasure. He didn’t know why or how? Maybe from whatever Slim injected in his soul? The heat was something different, something better. 

Black laid on his back and took his brother's cock. He couldn’t move at all. He was being dominated by his brother. This is wrong but felt so good. At least one good thing came out of this scenario.

Slim began to pound into Black. Something different was coming, he needed to let it out. His brother came but that didn’t mean he had to stop. He kept thrusting harder and deeper.

Black was in bliss. He came harder than he’s ever came before. He was over stimulated but knew he couldn’t stop his brother. Before he knew it, he was came once again. His eye lights rolled to the back of his head and turned to hearts. His mind was so fogged up. He was moaning in ecstasy until his brother stopped thrusting.

Slim as plunged deep inside Black. The tip swelled up keeping them stuck together. Something was coming out of his brother’s dick, he could feel it. It felt like a ball, some kind of thick round thing pushing it’s way through his entrance. Black winced in pain as it sent through and finally pushed further into his pussy. Not even realizing it was summoned, the egg plopped right in his womb. It was an egg.  **It was a fucking egg.**

 With the little movement Black had, he loved his head up and saw the first egg through his magic. Tears swelled at his sockets as he felt another egg push it’s way inside him. The eggs weren’t too big, but it still hurt. The only pleasure he got out of this was when an egg would be pushed against his g-spot. 

Slim felt so much bliss. This was a whole other level of release. It felt good to get rid of whatever was coming out of him. He knew his brother was uncomfortable but he didn’t care. He felt like butter, so soft and warm. 

He couldn’t hold back any moans. He was going from orgasm to orgasm. The eggs were coming in quickly. They were pushing against his spot much harder. He didn’t care how swollen his belly was getting, he didn’t care that he was being impregnated, he couldn’t care. All the cared about was his brother's cock and the pleasure.

After what felt like 15 minutes in heaven, the eggs stopped. The knot at the tip of Slim's dick swelled down. He pulled out leaving Black whining for more. His stomach mad whom look pregnant. Who knows how many eggs there are? Know one cared. 

Slim instinctively moved his other dick to Black's entrance. He thrust in easily, the vagina was already stretched and begging for more. 

The bed creaked and rocked back and forth from the force of Slim's thrusting. Black's mind was clearly gone at that point. He has being hit by so many orgasms. The pleasure took over his body and mind. He was nothing more than a sex doll at that point.

Slim slammed inside his brother's frail body and came inside. The cum washed over all the eggs, soaking them. He kept coming for the next three minutes. His magic and Black's was spilling out and onto the bed. 

By that point, Black had passed out. Slim wasn’t finished yet. He needed to make sure all those eggs were fertilized. He began ramming into Black once more. He kept doing so until he came. This time he felt more comfortable about the eggs being fertilized. They would have beautiful babies. He didn’t care if Black wanted them or what he thinks. Slim was the one in control, he was the alpha now.

The door to their room creaked opened. Slim turned around sharply to see an intruder in their house. Ink stood at the doorway, the cure in his hand. He stared at Slim with a thousand emotions he didn’t know he had. Fear was one of them.

**Author's Note:**

> So even though there weren't many votes, this was voted #1. There'll be a Swapcest oviposition, a Swapcest fluff, and a Black x Blue fluff with some smut :)


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